Field of the Invention
The invention relates to a method for recording (acquiring) a magnetic resonance data set of a target region of an object with a magnetic resonance scanner, wherein the target region contains at least one interfering object with a susceptibility difference from the rest of the target region that influences the homogeneity of the basic magnetic field, in particular a metal object and/or an air inclusion. The invention also relates to a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Magnetic resonance imaging is widely known and is in frequently used in clinical medical diagnosis. A basic field (B0 field) generated in the magnetic resonance scanner is as homogenous as possible in the imaging volume, and is used to align nuclear spins in a target region of a subject. Radio-frequency excitation (B1 field) excites the spins and the decay of the excitation is measured (detected) as a magnetic resonance signal. Usually, spatial resolution is obtained by means of gradient fields generated by a gradient coil arrangement, for example slice-selection gradients when radiating the radio-frequency excitation, phase-encoding gradients, and readout gradients. In order to obtain the best possible imaging quality, homogeneity of the B0 field and linearity of the gradient fields are required.
In this context, problems are caused by susceptibility differences in the target region which can form at the interfaces of objects in the target region, for example in the case of air inclusions and/or in the environment of metal, for example in the case of implants, prostheses, dental fillings and the like. The susceptibility differences disrupt the homogeneity of the basic field, which can in turn result in artifacts in the magnetic resonance image. Such artifacts depending on the magnetic resonance sequence used are identifiable from signal loss and/or distortion in the magnetic resonance image.
Methods have been suggested to reduce artifacts of this kind in magnetic resonance data sets, in particular in turbo-spin-echo (TSE)-based magnetic resonance sequences. Examples of these are methods such as WARP or SEMAT. A further method, which can be advantageous for imaging in the environment of disruptive objects, in particular metal objects, is the use of single-point imaging methods (SPI), for example as a RASP or SPRITE magnetic resonance sequence.
Since these magnetic resonance sequences read each k-space point at the same time, the dephasings induced by the susceptibility differences are the same at each point so that no distortion can occur in the image.
To date, no suitable solution for gradient-recalled-echo (GRE)-based magnetic resonance sequences, which enable shorter echo and repetition times than TSE-based magnetic resonance sequences, is known. In particular, due to the high T2* decay and the signal loss resulting therefrom, GRE sequences have been considered as unsuitable for imaging in the environment of objects, in particular metal objects.
An article by Jan-Henry Seppenwoolde et al., “Passive Tracking Exploiting Local Signal Conservation: The White Marker Phenomenon”, Magnetic Resonance in Medicine 50:784-790 (2003) suggests a novel approach to passive tracking of paramagnetic markers during endovascular interventions. The concept of this is to enable a positive contrast in the environment of small paramagnetic markers by selective dephasing of resonant unperturbed spins and simultaneous rephasing of magnetically perturbed spins. This should make it easier to locate the paramagnetic markers. The Seppenwoolde et al. method does not provide any advantages for normal clinical imaging.